Fixed bed Fisher-Tropsch catalysts based on Fe and Co are supplied typically as 1-3 mm tri-lobe extrudate in oxide form. This material is charged into reactor tubes, typically about 25-50 mm internal diameter and then reduced in-situ using a hydrogen-containing gas. The Fisher-Tropsch reactor tubes can be up to 12-14 meters in length, therefore loading pellets is time consuming and laborious. Furthermore, loading such tubes by pouring the particles in from the top inevitably results in breakage of a portion of the catalyst particles causing dust and an increase in the pressure drop through the tubes, which is undesirable. Before use, the catalyst is reduced. The reduction is exothermic and therefore it can be difficult to control the temperature of the catalyst, which can be degraded by overheating. The reduction can also cause catalyst volume shrinkage in the tubes, which can be significant, requiring topping up of the tubes, which again is time consuming.
These problems are not unique to fixed-bed Fischer-Tropsch reactors but also exist to some extent for other reactors where the catalyst is placed in tubes, such as methanol synthesis or methanol reforming, water-gas shift and the steam reforming of hydrocarbons.
Therefore there is a need to improve the efficiency of loading catalyst-filled tubes and activating the catalyst therein, particularly for reducible metal catalysts.
Wax-encapsulated Fischer-Tropsch catalysts are known for slurry-phase catalyst technology. For example, WO 2005/065824 A1 discloses a process to form pastilles, which have a diameter in the range 2-100 mm and a thickness in the range 1-10 mm, and an apparatus for making the pastilles. The process comprises mixing an active catalyst powder with a particle size in the range 1-200 microns with a hydrocarbon material in a low-shear jacketed blender at a temperature slightly above the congealing point of the hydrocarbon, and then making pastilles or flakes from the catalyst/hydrocarbon mixture while cooling the mixture to temperature below the congealing point of the hydrocarbon. Such pastilles are generally used in slurry phase reactors and are not suitable for tubular reactors.
WO 2006059148 A1 discloses a method for the preparation of an eggshell catalyst comprising the steps of; i) immersing shaped units of an oxidic support having a smallest unit dimension ≧0.5 mm in a solution of cobalt amine carbonate, ii) heating the solution to a temperature between 60 and 120 DEG C. to precipitate cobalt compounds onto the surface of the shaped units, iii) separating the resulting supported cobalt compounds from the remaining solution, and iv) drying the supported cobalt compounds. The cobalt compounds may be reduced to provide catalysts suitable for the hydrogenation of unsaturated compounds or the Fischer-Tropsch synthesis of hydrocarbons. For FT catalysts, the reduced cobalt catalyst may be protected by encapsulation of the catalyst particles with a suitable barrier coating such as a FT-hydrocarbon wax. However no details of the encapsulation process are given.
EP 2000206 A1 discloses a method of strengthening a particulate carrier or a catalyst or catalyst precursor having a particle size of at least 1 mm by adding one or more waxes to the particles. The preferred method involves a wax coating on an outer layer of the individual particles with removal of the superfluous wax. Similarly, EP 2000207 A1 discloses a method of strengthening a catalyst or catalyst precursor comprising a porous body having a size of at least 1 mm, and a catalyst or catalyst precursor material, the porous body along with the catalyst or catalyst precursor material having a porosity of at least 50% and an average pore size of more than 10 μm, by adding one or more waxes to the particles. Both EP 2000206 and EP 2000207 relate to coating individual catalyst particles and seek to improve their individual strength by filling the pores with the wax.
However these disclosures do not provide a suitable means to more efficiently load tubular reactors.